1. Field of the Invention
The present invention relates generally to tractoring mechanisms for use in wells. More particularly, the present invention relates to a mechanism that assists tractoring in wells having uniform and non-uniform surfaces by adjusting or adapting its configuration in response to the internal surface configuration of the wellbore, well casing, or pipe through which it is moved. Even more particularly, the present invention is particularly applicable to the field of borehole tractors for conveying logging and service tools in deviated or horizontal oil and gas wells, or in pipelines, where such tools may not readily be conveyed by the force of gravity.
2. Description of Related Art
U.S. Pat. No. 4,557,327 discloses a roller arm centralizer mechanism that is basically in the form of a four-bar mechanism. The disadvantage of this mechanism for tractoring is that the force required to push it through casing joints is several times higher than that required with the six-bar mechanism utilized in the present invention. U.S. Pat. No. 4,243,099 discloses a two-bar mechanism having motor positioned arms with bow springs causing rollers to maintain contact with the borehole wall surface. If used to assist tractoring systems, the rollers of this well tool mechanism will enter casing joints and other depressions and almost always become caught in most casing joints. U.S. Pat. No. 5,358,039 discloses a centralizer mechanism having a non-centered system of four-bar mechanisms with bow springs around them. This system will not allow tractoring systems to pass casing joints and changes of pipe diameter while simultaneously tractoring. U.S. Pat. No. 6,232,773 discloses a tractor vehicle that tows a support vehicle through a flexible coupling. This tractor mechanism employs linkage assemblies in the form of four-bar mechanisms, but does not offer the advantages of the present invention that is based on a six-bar mechanism. U.S. Pat. No. 5,848,479 presents another centralizer option, but does not offer the advantages of the present invention. Finally, the apparatus set forth in U.S. Pat. Nos. 5,794,703 and 5,184,676 are also based on four-bar linkage mechanisms that do not offer the advantages of the present invention.